The invention relates to a process for the asymmetric hydrogenation of furoimidazole derivatives of the general formula: ##STR3## in which R.sub.1 denotes a protective group which can be eliminated in a known manner, and R.sub.2 represents hydrogen or a protective group which can be eliminated in a known manner, with hydrogen in the presence of a homogeneous catalyst to give the corresponding diastereomeric dihydrofuroimidazole derivatives of the general formula: ##STR4## in which R.sub.1 and R.sub.2 have the above-stated meaning.
2. Background Art
Most of the known (+)-biotin syntheses pursue the aim of separating suitable precursors by racemate resolution methods, some of which are very elaborate and some of which use very costly resolving agents, and continuing the (+)-biotin synthesis with the resultant enantiomers (compare, for example, German Patent No. 2,058,248). According to European Patent No. 273,270, introduction of the correct configuration, that is to say (S) in the 3a and (R) in the 6a position of the biotin ring structure, was then achieved for the first time by asymmetric hydrogenation of corresponding furoimidazole derivatives with a classical hydrogenation catalyst, such as, rhodium on aluminium oxide. However, this process was unable to provide complete satisfaction in terms of the yield of desired diastereomer which was achievable.